Motores dc
By: Rick Bickle
11/7/2003
Motor control questions
Why do we need speed control? How is DC motor speed controlled? How is motor direction controlled? What circuits can be used?
Reasons for accurate speed control
Motor speed should be independent of load. Differential drive platforms need to synchronize wheel speed to go in a straightline.
Speed control with PWM
Pulse Width Modulation
Simple PWM circuit
U1A 74HC14A
U1B
1
2
3
4
74HC14A
D1 DIODE
U1C
MG1
6
1
5
1
2
2
74HC14A
D2 DIODE
R2 POT
9
MOTOR DC
U1D
8
C1 CAP NP
3
74HC14A
U1E
11
10
74HC14A U1F
13
12
74HC14A
H-Bridge motor driver circuit
CIRCUIT INPUTS A 0 1 1 0 X B 0 1 0 1 X C 00 0 0 1 Output N Channel Brake P Channel Brake Forward Reverse Motor Off G D
M1
V+
R1 4.7K
R2 4.7K
S
Q1 IRF9510
S
Q2 IRF9510
G D
+5
DC Motor
+5
D
14
D
Q3 IRF510
Q4 IRF510
G
R3 4.7K
G S
Q6 2N2222
R4 4.7K
6
14
U1B 74HC08
S
Q5 2N2222
Input A
1
4
3
2
U1A 74HC08
Input B
5
7
7
V+
+5
R5
+5
D
14
9
U1C74HC08
Q7 IRF510
4.7K
U1C 74HC08
G S
R6 4.7K
Q8 2N2222
14
9
8
10
8
10
PWM Input
7
7
Optical encoder circuit
11
U27E 74HC14 10
13
U27F 74HC14 12
R11
C14
10K
0.001uF
R6 4.7K
L. Motor
1
Tach Input A
U27A 74HC14 2
U27B 74HC14 4
U6A
1
3
9
8
10
2
R7 4.7K
3
Tach Input B
74HC86
R10
U6C 74HC86
C15
D3 5V
R84.7K
R. Motor
D4 5V
U27C 74HC14 6
10K
0.001uF
U6B
4
6
13
11
5
Tach Input A
12
R9 4.7K
9
Tach Input B
U27D 74HC14 8
5
74HC86
U6D 74HC86
D5 5V
D6 5V
Motor control diagram
H-BRIDGE CIRCUIT DC MOTOR
PROCESSOR
ENCODER
Control systems
What is a control system? What are some examples? What are the types of control systems? How are controlsystems represented?
Open loop control systems
The output of the plant does not affect the input. (No feedback) Less common today than closed loop control systems. Examples include:
Stereo volume control Electric drill speed control
Open loop control system
INPUT
GAIN
PLANT
OUTPUT = INPUT X GAIN
Closed loop control systems
Use a measurement of output to control the input(Feedback) Examples include:
Air conditioning thermostat Automobile cruise control
Closed loop control system
INPUT
SUM
GAIN
FEEDBACK
PLANT
OUTPUT = (INPUT – OUTPUT) X GAIN
Motor control diagram
H-BRIDGE CIRCUIT DC MOTOR
PROCESSOR
ENCODER
PID Closed loop control system
PID controls the gain portion of the closed loop control system. PID algorithms adjust the gainto the plant based on several characteristics of the feedback, not just the current value.
PID control system diagram
P GAIN
INPUT
SUM
I GAIN
SUM
PLANT
D GAIN
FEEDBACK
OUTPUT = (INPUT – OUTPUT) X (P GAIN + I GAIN + D GAIN)
Sample PID output chart
Set point Rise time Overshoot Settling time Peak time Overdamped Underdamped
PID implementation
What is themathematics of PID? How is it programmed? What are some common problems? How is the PID behavior optimized?
PID variables
Error term P – Proportional gain I – Integral gain D – Derivative gain
Error term
The error term is derived by subtracting the feedback (motor speed) from the set point (set speed). This is the error in terms of a number of encoder counts per unit time.
P GAIN
INPUTSUM
I GAIN
SUM
PLANT
D GAIN
FEEDBACK
Proportional term
Simple proportional coefficient Kp is multiplied by the error term. Provides linear response to the error term.
P GAIN
INPUT
SUM
I GAIN
SUM
PLANT
D GAIN
FEEDBACK
Integral term
Integral coefficient Ki is multiplied by the error term and added to the sum of all previous integral terms. Provides...
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